1. Introduction
This invention relates to acrylic polymers having improved optical transparency and to photolithographic compositions containing said polymers. More particularly, this invention relates to novel photolithographic coating compositions having improved optical transparency, especially at exposures of 193 nm. The composition is prepared by a direct process that avoids resin recovery as an intermediate step.
2. Description of the Prior Art
Photoresists are photosensitive films used for transfer of images to a substrate. They form negative or positive images. Following the coating of a liquid photoresist composition onto a substrate, the coating is dried and exposed through a patterned photomask to a source of activating energy such as ultraviolet light to form a latent image in the photoresist coating. The photomask has areas that are opaque to activating energy and other areas that are transparent to activating radiation. The opaque and transparent areas define an image to be transferred to an underlying substrate. A relief image is formed in a photoresist film following exposure through the mask by development of the latent imaged pattern in the coating. The use of photoresists is generally described, for example, by DeForest, Photoresist Materials and Processes, McGraw Hill Book Company, New York (1975), and by Moreau, Semi-conductor Lithography, Principles, Practices and Materials, Plenum Press, New York (1988).
Known photoresists can provide features having resolution and size sufficient for many existing commercial applications. However, for many other applications, the need exists for new photoresists that can provide highly resolved images of sub-micron dimension upon exposure and inclusive of exposure to shorter wavelength activating radiation such as 193 nm. Various attempts have been made to alter the make-up of photoresist compositions to improve performance and functional properties. Among other things, a variety of novel photoactive compounds have been reported for use in photoresist compositions. See, for example, U.S. Pat. No. 4,450,360 and European Application No. 0 615 163, both incorporated herein by reference.
Recently, several acid catalyzed chemically amplified resist compositions have been developed such as those disclosed in U.S. Pat. Nos. 4,968,581; 4,883,740; 4,810,613; and 4,491,628; Canadian Patent Application No. 2,001,384 and Nalamasu et al., "An Overview of Resist Processing for Deep-UV Lithography", J. Photopolym. Sci. Technol. 4, 299 (1991). These resists possess improved sensitivity. The chemically amplified resist composition, in one embodiment, comprises a photosensitive acid generator and an acid sensitive polymer. The polymer has acid sensitive side chain (pendant) groups which are bonded to the polymer backbone and are reactive towards a proton. Upon image-wise exposure to radiation, the photoacid generator produces a proton. The resist film is heated causing the photolitically generated proton to cleave the pendant group from the polymer backbone. The released proton is not consumed in the cleavage reaction and catalyzes additional cleavage reactions thereby chemically amplifying the photochemical response of the resist. The cleaved polymer is soluble in polar developers such as alcohol and aqueous base while the unexposed polymer is soluble in non-polar organic solvents. Thus, though the photoresist is primarily used as a positive resist, the resist can produce positive or negative images conforming to the mask dependent upon the selection of the developer and radiation dose.
An alternative chemically amplified resist composition is one containing the photosensitive acid generator and an acid sensitive cross-linking agent. Upon image-wise exposure to activating radiation, the photoacid generator produces a proton. The resist film is heated and the proton released by heating activates the cross-linking agent which reacts with certain functional groups on the polymer backbone curing the same. Again, the proton is not consumed by activation of the cross-linking agent and catalyzes additional cross-linking reactions thereby chemically amplifying the photochemical response of the resist. The cross-linked polymer is soluble in organic solvents and insoluble in alcohol and aqueous base while the unexposed polymer remains soluble in such aqueous base developers. For this reason, though the resists are primarily used as negative resists, they can produce positive or negative images of the mask dependent upon the selection of the developer.
Preferred chemically amplified photoresists comprise an admixture of a photoactive compound and a resin binder that traditionally comprises a copolymer containing phenolic units though more recently developed systems use compolymers of phenolic and non-phenolic units. For example, one preferred group of such copolymers has acid labile groups substantially, essentially or completely only on non-phenolic units of the copolymer. For example, one copolymer binder has repeating units x and y of the following formula: ##STR1## where the hydroxyl group is present at either the ortho, meta or para positions throughout the copolymer, and R' is substituted or unsubstituted alkyl having 1 to about 18 carbon atoms, more typically 1 to about 8 carbon atoms. Tert-butyl is a generally preferred R' group. An R' group may be optionally substituted by e.g. one or more halogen (particularly F, Cl or Br), C.sub.1-8 alkoxy, C.sub.2-8 alkenyl, etc. The units x and y may be regularly alternating in the copolymer, or may be randomly interspersed through the polymer. Such copolymers can be readily formed. For example, for resins of the above formula, vinyl phenols and a substituted or unsubstituted alkyl acrylate such as t-butylacrylate and the like may be condensed under free radical conditions as known in the art. The substituted ester moiety, i.e. R'--O--C(.dbd.O)--, moiety of the acrylate units, serves as the acid labile group of the resin that will undergo photoacid induced cleavage upon exposure of a coating layer of a photoresist containing the resin. Typically, the copolymer will have a M.sub.w in excess of 10,000 Daltons and often up to about 50,000 Daltons, more typically, from about 15,000 to about 30,000 Daltons with a molecular weight distribution of about 3 or less, more preferably a molecular weight distribution of about 2 or less.
The presence of the phenolic units in the above resin results in substantial adsorption of activating radiation, particularly at the near and deep ultraviolet wavelengths. As a consequence, though the acrylate portion of the polymer improves optical transparency, especially to the shorter wavelengths of activating energy, optical transparency of the resin is not fully optimized as a consequence of the phenolic units in the polymer.
A copolymer of an alkyl acrylate such as t-butyl acrylate or t-butyl methacrylate and a vinyl alicyclic such as a vinyl norbornane or vinyl cyclohexanol compound, are also known. Such copolymers may also be prepared by free radical polymerization. Exemplary polymers and photoresists using such polymers are disclosed in U.S. Pat. No. 5,492,793 incorporated herein by reference. Copolymers of the type described in this patent possess enhanced sensitivity as a consequence of the copolymerized acrylate units in the polymer. It is known in the art that acrylic resins have desirable optical transparency properties, particularly at the shorter wavelengths. Good optical transparency in a photoresist is essential for certain uses.
Because of their excellent optical transparency, acrylic polymers of the type described above have been used as binders in the formulation of many photoresist compositions, especially for those photoresists where optical transparency is essential. However, it is known that the presence of the aromatic ring reduces optical transparency at certain wavelengths, especially at the shorter wavelengths such as 193 nm and below. In addition, it has been found that the acrylic polymers, though optically transparent when made, have a tendency to yellow upon standing, thus reducing the optical transparency of the resist as it ages. Antioxidants have been added to photoresists to prevent yellowing, but have been found to be ineffective or have been found to interfere with the photolithographic chemistry of the photoresist composition.